1. Field of the Invention
The present invention relates to a device for fixing a gas showerhead or target to an RF or DC electrode in a plasma processing system. More particularly, the present invention relates to a technique to fix the gas showerhead used in plasma assisted chemical vapor depositions or dry etching systems, or the target used in plasma assisted sputter deposition systems to improve the thermal conductance from the showerhead or target to the RF or DC electrode and the gas distribution below the showerhead.
2. Description of the Related Art
Plasma assisted wafer processing technique is well-accepted method in semiconductor device manufacturing industry. Two major plasma generation techniques are the RF (Radio Frequency) plasmas and the DC (Direct Current) plasmas with parallel plate configuration, where RF or DC electrode is in parallel with a wafer surface. In most such plasma assisted wafer processing reactors there is a gas showerhead or target plate fixed to the RF or DC electrode. Generally, a showerhead is fixed to the RF or DC electrode in chemical vapor depositions and dry etching reactors, while a target plate is fixed to the RF or DC electrode in sputter deposition reactors.
Magnetic field applied plasma processing reactors are currently being used in dry etching applications.
At present, the showerhead or target plate is fixed to the RF or DC electrode by using bolts or a ring-shaped flange placed around the showerhead or target. This method of fixing poses two major problems. The first problem is insufficient thermal conductance between the showerhead or target and the electrode, and the second problem is non-uniform gas distribution below the showerhead. These problems are explained with reference to FIGS. 18 and 19.
For the ease of explanation, for example, a point-cusp magnetic field applied RF plasma processing system used for dry etching applications is selected (for example, JP-A-07-335635). A cross sectional view of the dry etching reactor 100 is shown in FIG. 18, in which plasma is generated by capacitively coupled mechanism. This plasma processing system is comprised of a top electrode 101, lower electrode 102, insulating materials 103 (103a, 103b) and 104 to electrically isolate electrodes 101 and 102 from the rest of the reactor 100, and a showerhead 105. The top electrode 101 is made of a metal usually aluminum and is cooled by flowing liquid through canals 106 made within the top electrode 101. The cooling is important in order to reduce or maintain a constant temperature at the gas showerhead 105.
There is a plurality of magnets 122 separately arranged on the outer surfaces of top electrode 101. The magnets 122 are arranged in a non-circular configuration with respect to a center of said top electrode 101. The magnets 122 are arranged in an orthogonal configuration along perpendicular lines such that a polarity of each of the magnets facing the inside of the reactor is opposite to that of linearly adjacent magnets and the same as diagonally adjacent magnets. These magnets 122 generate a magnetic field with closed magnetic fluxes near the inner surfaces of the top electrode 101.
In addition, there is gas reservoir 107 within the top electrode 101. A plurality of gas inlets 108 is made from this gas reservoir 107 to the inside of the reactor 100. Use of the gas showerhead 105 is very important for most of the dry etching processing in order to obtain a uniform etch rate on the substrate surface.
The top electrode 101 is supplied a RF power from a RF generator 109 via a matching circuit 110. Similarly, another RF current from a RF generator 111 is supplied to the lower electrode 102 via a matching circuit 112. The frequencies of RF currents applied to the electrodes 101 and 102 are not important for the present invention. Therefore, here, the frequencies are not discussed in detail.
In addition, JP-A-2001-267297 and JP-A-2001-267295 are cited as the other related arts.
Generally, the showerhead 105 is made of a semiconductor (e.g. Si) or a dielectric material (e.g. Quartz). The showerhead 105 is usually fixed to the top electrode 101 by using only its outermost region, typically about 2–5 mm wide band at the edge. The gas showerhead 105 shown in FIG. 18 is fixed to the top electrode 101 by using a ring-shaped dielectric ring 103a, which is fixed to another dielectric ring 103b using a plurality of bolts 113. Or one can fix the showerhead 105 to the top electrode 101 directly by the bolts 113 as shown in FIG. 19 without using the dielectric ring 103a. In either way, the showerhead 105 is attached to the top electrode 101 only by its outermost region.
Even though the lower surface of the top plate 101 and the upper surface of the showerhead 105 are taken as planar as possible, the showerhead 105 makes contact with the top electrode 101 only at its outermost region. Generally, the central region of the showerhead 105 bends due to its weight as shown in FIG. 19. Due to this bending, a very thin gas reservoir 114 is made between the lower surface of the top electrode 101 and the upper surface of the showerhead 105. This causes two problems.
The first problem is the reduction of thermal conductance between the showerhead 105 and the top electrode 101. During the plasma processing, the plasma heats the showerhead 105. This heat must be efficiently transferred to the top electrode 101 in order to reduce the temperature rise of the showerhead 105. However, since the showerhead 105 is attached to the top electrode 101 only by its edges, heat transfer occurs only through the edge of showerhead 105. This causes a higher temperature gradient between the center and the edge in the showerhead 105. Maintaining an almost constant temperature on the entire surface of showerhead 105 is of important for most of the dry etching applications. Because non-uniform temperature on the showerhead 105 changes the chemistry of process gas below the showerhead 105, it adversely affects the process on the wafer surface.
The second problem is that due to the formation of the thin gas reservoir 114 between the top electrode 101 and the showerhead 105, the gas inlets 108 from gas reservoir 107 to reactor 100 discontinue. This causes process gas leak into the thin-gas-reservoir 114. The gas pressure within the thin-gas-reservoir 114 tends to increase at its center. Because of this reason, a higher amount of process-gas begins to come from the central region of the showerhead 105. This changes the uniformity of process-gas flux into the reactor 100 and thereby the uniformity of gas chemistry below the showerhead 105. This matter results in non-uniform etch rate on the surface of the wafer 115.
Even though the above two problems are explained using a dry etching reactor, the same problems exist in most of the other plasma assisted wafer processing reactors with or without the magnetic field assistance.